The present invention relates to a laser scanning optical system to be employed in a color imaging apparatus such as a color laser beam printer or the like.
An example of a conventional laser scanning optical system is shown in FIG. 5. The scanning optical system is provided with a laser source 11 for emitting a laser beam, a collimating lens 12 for converting the laser beam emitted by the laser source into a parallel light beam. The parallel light beam is incident on a cylindrical lens 13 which converges the incident beam only in one direction so that a line-like image is formed in the vicinity of a polygon mirror 14. The polygon mirror 14 is driven to rotate, and the incident beam is deflected in accordance with the rotation of the polygon mirror 14. In this specification, a plane including the principal ray of the deflected beam is defined as a main scanning plane. The deflected laser beam passes through an f.theta. lens 15 and forms a beam spot on a photoconductive drum 16. The beam spot formed on the photoconductive drum 16 scans (moves on the photoconductive drum 16) in a direction parallel to a rotation axis Ax of the photoconductive drum 16 (a main scanning is performed). The direction in which the beam spot moves, i.e., the direction parallel to the rotation axis Ax of the photoconductive drum 16 will be referred to as a main scanning direction hereinafter.
During the main scanning, the photoconductive drum 16 is rotated (an auxiliary scanning is performed). As the main scanning and auxiliary scanning are performed, a two-dimensional latent image is formed on the photoconductive drum 16. The f.theta. lens 15 functions such that a constant angular speed of the incident beam is converted into a constant speed of the beam spot on the photocondutcive drum 16.
In the conventional scanning optical system, even if the intensity of the laser beam emitted by the laser source is constant, the intensity of the beam incident on the photocondutcive drum 16 may vary depending on the position in the main scanning direction.
In such a case, quality of the two-dimensional image may be lowered. That is, when the latent image is developed (toner is applied) and transferred on a recording sheet, a portion of the transferred image which should have even density distribution over the two-dimensional image area, may be formed to have uneven density distribution.
FIG. 4A shows an intensity distribution characteristic of the laser beam on the photoconductive drum 16 in the main scanning direction. In FIG. 4A, Ps denotes a start position of an effective scanning area on the photoconductive drum 16, and Pe denotes an end position of the effective scanning area. Pc denotes the center of the effective scanning area, which coincides with a position where an optical axis Ox of the f.theta. lens 15 intersects the photoconductive drum 16.
As shown in FIG. 4A, the intensity of the beam is greatest at the center of the scarring area, and decreases towards the peripheral portions.
Such a characteristic is caused by a relationship between an incident angle of the laser beam incident on the reflection surface of the polygon mirror 14 and a reflection ratio thereof, and by a relationship between an incident angle of the laser beam incident on the f.theta. lens 15 and a transmission ratio thereof.
This characteristic is inconspicuous when the two-dimensional image is a monochrome image. However, such a characteristic would be conspicuous when the scanning optical system is applied to a color image device.
It may be possible to compensate the uneven intensity distribution characteristic as shown in FIG. 4A by providing a filter of which the density distribution is adjusted to cancel the unevenness distribution of the intensity of the beam on the photoconductive drum 16. However, since the density distribution of such a filter should be determined based on the configuration of the f.theta. lens 15, it is difficult to design and manufacture such a filter. As a result, such a filter would become relatively expensive, and therefore it would be difficult to provide an inexpensive laser scanning optical system for a color imaging apparatus.